1. Field of the Invention
The disclosures herein relate to a touch panel.
2. Description of the Related Art
A touch panel is an input device that allows input to be directly entered into a display, and is placed in front of the display. Touch panels are widely used in various applications because of their capacity of allowing direct inputting based on visual information provided by the display.
A resistive-type touch panel is widely known in the art. The resistive-type touch panel includes an upper electrode substrate and a lower electrode substrate having respective transparent conductive films. These substrates are arranged such that the corresponding transparent conductive films face each other. When pressure is applied to a point on the upper electrode substrate, the transparent conductive films are brought into contact with each other, thereby allowing the position of the pressed point to be detected.
The resistive-type touch panel is classified into a four-wire type and a five-wire type. In the four-wire type, an X-axis electrode is disposed on one of the upper electrode substrate and the lower electrode substrate, and a Y-axis electrode is disposed on the other substrate. In the five-wire type, an X-axis electrode and a Y-axis electrode are both disposed on the lower electrode substrate while the upper electrode substrate serves as a probe for detecting voltage (see Japanese Patent Application Publication No. 2004-272722 and Japanese Patent Application Publication No. 2008-293129, for example).
In the following, a description will be given of a five-wire-type touch panel by referring to FIG. 1 and FIG. 2. FIG. 1 is an oblique perspective view of a five-wire-type touch panel. FIG. 2 is a schematic view illustrating a cross section of the five-wire-type touch panel.
A five-wire-type touch panel 200 includes an upper electrode substrate 210 formed of a film having a transparent conductive film 230 formed on a surface thereof, and further includes a lower electrode substrate 220 formed of glass having a transparent conductive film 240 formed on a surface thereof. The upper electrode substrate 210 and the lower electrode substrate 220 are arranged with spacer 250 intervening therebetween such that the transparent conductive film 230 and the transparent conductive film 240 face each other. The five-wire-type touch panel 200 is electrically coupled to a host computer (not shown) through a cable 260.
In the touch panel as described above, information is entered through the event that the transparent conductive film 230 of the upper electrode substrate 210 comes in contact with the transparent conductive film 240 of the lower electrode substrate 220. There may be a case, however, in which the transparent conductive film 230 and the transparent conductive film 240 fail to come in contact with each other when the touch panel is pressed at a point close to the spacer 250 with the same amount of force that may be used when pressing the touch panel around the center thereof. In the touch panel, the transparent conductive film 230 and the transparent conductive film 240 come in contact with each other when the film constituting the upper electrode substrate 210 is warped upon being pressed by a finger or the like. However, constant interval between the substrates is maintained by the spacer 250 in the vicinity of peripheral edges where the spacer 250 is provided. Because of this, a larger degree of warping than in the center area is required at the periphery of the touch panel in order to bring the transparent conductive film 230 and the transparent conductive film 240 in contact with each other.
A decorative film may be attached to the upper electrode substrate 210, which makes the upper electrode substrate 210 more difficult to warp. The trend to use such a decorative film is gathering momentum. As illustrated in FIG. 3, a decorative film 270 is attached to the upper electrode substrate 210 through an adhesive layer 271 made of acrylic resin or the like. An integrated structure including the decorative film 270, the adhesive layer 271, and the upper electrode substrate 210 needs to be warped by a finger press. However, this integrated structure is thick and hard to bend. Especially in the vicinity of the spacers 250 (i.e., positions near the places where the spacer 250 is provided), an extremely strong force may be required to bring the transparent conductive film 230 and the transparent conductive film 240 into contact with each other.
In this manner, a pressing force required to bring the transparent conductive film 230 and the transparent conductive film 240 into contact with each other may vary between the central area of the touch panel and the peripheral area thereof. As a result, information may not be entered in the vicinity of the peripheral edges of the touch panel if the same amount of force that is used to press the center area of the touch panel with a finger to enter information is used in the peripheral area. Such area-dependent variation in the amount of force required to press the touch panel to enter information may result in information failing to be properly entered with sufficient accuracy. The entirety of the decorative film 270 is formed of a transparent resin material such as PET (polyethylene terephthalate) or the like. Further, a black decorative area 272 is provided for the purpose of concealing the spacer 250 at each of the four edges of the touch panel.
Accordingly, there may be a need for a touch panel in which an electrode-to-electrode contact is made in the vicinity of peripheral edges of the touch panel where a spacer is provided, despite the use of the same amount of force that is used in the central area of the touch panel, thereby allowing information to be entered by the same force across the entire screen.